Imaging control apparatus, ophthalmic imaging apparatus, imaging control method, and program

ABSTRACT

An imaging control apparatus capable of causing a first imaging unit to capture a fundus image of a subject&#39;s eye and causing a second imaging unit to capture a tomographic image of the subject&#39;s eye includes a control unit configured to cause the first imaging unit to capture an image of the fundus of the subject&#39;s eye in a limited area smaller than the fundus image, a detection unit configured to detect a positional deviation of the fundus of the subject&#39;s eye based on an image of the area acquired according to the control, and a correction unit configured to correct an image capturing position of the tomographic image captured by the second imaging unit based on the detected positional deviation.

BACKGROUND

Various types of ophthalmic apparatuses employing optical apparatusesare used. For example, optical apparatuses for observing eyes includeanterior eye imaging apparatuses, fundus cameras, and confocal scanninglaser ophthalmoscopes (SLOs). Especially, an optical coherencetomography apparatus (hereinafter, referred to as OCT apparatus) canacquire high-resolution tomographic images of a sample, and theapparatus has become an essential ophthalmic apparatus used inoutpatient clinics specializing in retina care.

The OCT apparatus enables high-sensitivity eye measurement byirradiating a sample with low coherent light, and using an interferencesystem of the reflected light from the sample. Further, the OCTapparatus can acquire high-resolution tomographic images by scanning thesample with the low coherent light. Since the OCT apparatus can capturehigh-resolution tomographic images of a retina in a fundus of asubject's eye, the OCT apparatus is widely used for an ophthalmicdiagnosis for retinas, and the like.

In the ophthalmic diagnosis for retinas, a tomographic image of aretina, which is referred to as a B scan image, captured by an OCTapparatus are generally used. The B scan image is acquired by performingscanning in a depth direction (Z direction) of the retina, which isreferred to as A scan, in an X direction for a plurality of times. Whencomparing with an image of a conventional fundus camera, a state of aninside of the retina can be observed from the B scan image, the B scanis an innovative technique for observing a lesion inside of the retina,especially, for observation of macular degeneration, a macular hole, andthe like.

The B-scan image is acquired by only one scanning. However, by scanningthe same point for a plurality of times and superimposing the scannedimages, a sharp image with less noise can be acquired. However, humaneyes move involuntary, which is referred to as involuntary eye movementduring fixation. Due to the movement, a target retina involuntary moveswhile the same point is geometrically scanned, and accordingly, it isdifficult to superimpose many images.

United States Patent Application Publication No. 2010-0157311 discussesa device for capturing a plurality of B-scan images in the Y directionto acquire a three-dimensional retinal image is developed. Theacquisition of the three-dimensional retinal image contributes to theobservation of the extent of a lesion and layers in a retina, especiallyto the observation of the ganglionic layer of optic nerve that causesglaucoma. However, in the observation, due to the movement of theeyeball, the three-dimensional retinal image may be distorted. To solvethis issue, a technique referred to as tracking has been developed. Inthe tracking technique, a fundus image is captured while tomographicimages of the retina are captured, and a feature point such as a branchpoint of a blood vessel in the fundus image is focused. When the featurepoint moves, the movement is determined to be the movement of theretina, and the scanning position of the OCT apparatus is changed.

To accurately measure the movement of the eyeball using the fundusimage, it is required to perform the processing of extracting thefeature point from the fundus image, searching and detecting the featurepoint in the images to be processed, and calculating an amount of themovement at a high speed. As the feature point in the fundus image, amacula, an optic papillary portion (hereinafter, referred to as an opticpapilla), or the like is used. In an affected eye, the macula and theoptic papilla are often unclear, and consequently, a blood vessel can beused as the feature point in fundus image. Japanese Patent ApplicationLaid-Open No. 2001-70247 discusses a method for extracting a featurepoint of a blood vessel.

The apparatuses for capturing a fundus image include fundus cameras andscanning ophthalmoscopes. The fundus camera acquires a fundus image ofthe whole area at one time. The scanning ophthalmoscope acquires afundus image by scanning the fundus with a beam. The scanningophthalmoscope includes the SLO and line-scanning laser ophthalmoscopy(LSLO). The SLO irradiates a laser spot on a fundus, and scans thefundus with a laser beam. The LSLO irradiates and scans a fundus with alinear laser beam. The scanning ophthalmoscopes are considered to beable to provide high-quality (high-resolution, high-luminance) imageswhile its image capturing time is longer than that of the fundus camera.Detailed configuration of the LSLO is discussed in U.S. Pat. No.4,768,874 and United States Patent Application publication No.2003-0231285. Generally, to measure the movement of an eyeball, afeature point is to be detected. Consequently, a scanning ophthalmoscopecapable of performing high-quality continuous shooting is used.

However, in the simultaneous imaging by the OCT apparatus for trackingand by the scanning ophthalmoscope described above, the imaging light ofthe OCT apparatus and the imaging light of the scanning ophthalmoscopesimultaneously enters the eye to be observed. Then, the light amountentering the eye increases. On the other hand, the upper limit of theincident light amount allowed for the eye observation is determined bythe International Organization for Standardization (ISO), and otherorganizations. Accordingly, the imaging light amount of the OCTapparatus is to be adjusted to a low amount. This results indeterioration in image quality such as decrease in signal-to-noise (S/N)ratios.

SUMMARY

According to some embodiments of the present invention, an imagingcontrol apparatus capable of causing a first imaging unit to capture afundus image of a subject's eye and causing a second imaging unit tocapture a tomographic image of the subject's eye includes a control unitconfigured to cause the first imaging unit to capture an image of thefundus of the subject's eye in a limited area smaller than the fundusimage, a detection unit configured to detect a positional deviation ofthe fundus of the subject's eye based on an image of the area acquiredaccording to the control, and a correction unit configured to correct animage capturing position of the tomographic image captured by the secondimaging unit based on the detected positional deviation.

Further features and aspects of the embodiment will become apparent fromthe following detailed description of exemplary embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, andfeatures, together with the description, serve to explain the principlesof the invention.

FIG. 1 is a view illustrating a configuration of a fundus imagecapturing apparatus according to an exemplary embodiment of the presentinvention.

FIG. 2 is a view illustrating a configuration of an OCT unit.

FIG. 3 is a view illustrating a configuration of a control device.

FIG. 4 is a flowchart illustrating processing performed by a controlunit.

FIG. 5 is a view illustrating an example of branch points of bloodvessels.

FIG. 6 is a view illustrating an example of typical movement of a humaneye.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects will be describedin detail below with reference to the drawings.

FIG. 1 illustrates a configuration of a fundus image capturing apparatus(ophthalmic imaging apparatus) according to an exemplary embodiment ofthe present invention. With reference to FIG. 1, a scanning laserophthalmoscope (SLO) for implementing a fundus observation method isdescribed. In FIG. 1, a laser light source 101 can be a semiconductorlaser or a super luminescent diode (SLD) light source. To reduce glaregiven to a subject and to maintain a resolution in the fundusobservation, it is preferable to use a laser light source of awavelength within the near-infrared region of 700 nm to 1000 nm. In thepresent exemplary embodiment, it is assumed that a semiconductor laserof the wavelength of 780 nm is used, and a light amount can be changedby control voltage.

A laser beam emitted from the laser light source 101 is collimated by acollimator lens 102, passes through a hole at the center of a perforatedmirror 103, and further passes through an SLO-X scanner 104 and an SLO-Yscanner 105. The laser beam further passes through abeam splitter 106and an eyepiece lens 107, and enters a subject's eye 108. According tothe present exemplary embodiment described below, an eye axis directionis defined as a Z coordinate, a horizontal direction with respect to afundus image is defined as an X coordinate, and a vertical directionthereof is defined as a Y coordinate.

The beam entered the subject's eye 108 irradiates the fundus in adot-like state. The beam is reflected or scattered on the fundus of thesubject's eye 108, passes through the same optical path and returns tothe perforated mirror 103. The reflected or scattered light is reflectedby the perforated mirror 103, and received by an avalanche photodiode(APD) 110 via a lens 109. Then, in the APD 110, a signal proportionateto a reflection/scattering strength at the point on the fundus isacquired. In addition, the SLO-X scanner 104 and the SLO-Y scanner 105perform raster scan, and a two-dimensional image of the fundus can beacquired.

With reference to FIG. 2, an optical coherence tomography (OCT) unit 115illustrated in FIG. 1 is described. FIG. 2 illustrates a configurationof the OCT unit 115. The OCT unit 115 divides the low-coherence lightinto reference light and observation light. The OCT unit 115superimposes the observation light via the subject's eye 108 and thereference light via a reference object to generate interference light,and outputs a signal by dispersing the interference light. The dispersedsignal is input into a central processing unit (CPU) 301 describedbelow. The CPU 301 analyzes the dispersed signal and forms a tomographicimage or a three-dimensional image of the fundus.

In FIG. 2, a low-coherence light source 201 includes a broadband lightsource that can output the low-coherence light. In the present exemplaryembodiment, an SLD is employed as the broadband light source. Thelow-coherence light includes the light of a wavelength range in thenear-infrared region, and has a coherence length of several tens ofmicrometers. For example, the low-coherence light has a wavelengthwithin the range from about 800 nm to 900 nm.

The low-coherence light output from the low-coherence light source 201is guided to an optical coupler 203 via an optical fiber 202. Theoptical fiber 202 typically employs a single mode fiber. The opticalcoupler 203 divides the low-coherence light into the reference light andthe observation light. The reference light generated by the opticalcoupler 203 is guided by the optical fiber 204 and collimated by acollimator lens 205. Then, the collimated reference light passes througha glass block 206 that functions as a dispersion compensation means forcompensating the dispersion characteristics of the reference light andthe observation light, and is reflected by a reference mirror 207. Thereflected reference light passes through the same optical path, andenters the optical fiber 204.

The reference mirror 207 can move in the direction the reference lightproceeds. Accordingly, adjustment of the length of the eye axis of thesubject's eye 108, and a distance between the eyepiece lens 107 and thesubject's eye 108, and the like can be performed, and distances of thereference light and the observation light are also adjusted.

Meanwhile, the observation light generated by the optical coupler 203 istransmitted through an optical fiber 208, an OCT-X scanner 113, and anOCT-Y scanner 112 to the eyepiece lens 107 illustrated in FIG. 1. Theobservation light is reflected and scattered on the retina of thesubject's eye 108 and becomes to signal light. The signal light entersthe optical fiber 208 again. The signal light guided to the opticalcoupler 203 via the optical fiber 208 interferers with the referencelight, and collimated by a collimator lens 210 via the optical fiber209. Then, the light is dispersed by a diffraction grating 311, and animage is formed on a one-dimensional sensor 213 by a lens 212. Theone-dimensional sensor 213 can be a charge coupled device (CCD) sensor,a complementary metal-oxide semiconductor (CMOS) sensor, or the like.With this configuration, the signal generated by dispersing theinterference light can be acquired from the one-dimensional sensor 213.

With reference to FIG. 1, a scan mechanism in the fundus image capturingapparatus according to the present exemplary embodiment is described.The observation light from the OCT unit 115 is collimated by acollimator lens 114, and passes through the OCT-X scanner 113 and theOCT-Y scanner 112. Then, the observation light is reflected by a mirror111 and a beam splitter 106, passes through the eyepiece lens 107, andenters the subject's eye 108. The observation light entered thesubject's eye 108 is reflected and scattered on the fundus, passesthrough the same optical path, and returns to the OCT unit 115.

A control device for controlling the fundus image capturing apparatusillustrated in FIG. 1 is described with reference to FIG. 3. FIG. 3 isillustrates a configuration of the control device. In FIG. 3, the CPU301 is connected with a display device 302, a main storage device 303(i.e., a random access memory (RAM)), and a program storage device 304(i.e., a read-only memory (ROM)). The CPU 301 is also connected with aone-dimensional sensor interface 306, an APD interface 307, and an SLOlight source digital-to-analog (D/A) converter 314. The one-dimensionalsensor interface 306 is used to input data, which is an output of theOCT unit 115, in a one-dimensional sensor 213. The APD interface 307 isused to input data of the APD 110, which is an output in the SLO method.The SLO light source D/A converter 314 generates voltage for controllingthe strength of the light source (laser light source 101) in the SLOmethod.

The CPU 301 is further connected with a SLO scanner control circuit 308and an OCT scanner control circuit 311 functioning as scannercontrollers. The SLO scanner control circuit 308 controls the SLO-Xscanner 104 using an SLO scanner driver (X) 309, and also controls theSLO-Y scanner 105 using an SLO scanner driver (Y) 310. The SLO scannercontrol circuit 308 controls a scan center position in the Y direction,a scan width in the Y direction, and a scan speed, in response to aninstruction from the CPU 301. Further, the CPU 301 can identify a scanposition of the laser beam based on an output from the SLO scannercontrol circuit 308.

The OCT scanner control circuit 311 controls the OCT-X scanner 113 usingan OCT scanner driver (X) 312, and also controls the OCT-Y scanner 112using an OCT scanner driver (Y) 313. The OCT scanner control circuit 311controls the scan center positions in the X direction and the Ydirection, the scan widths in the X direction and the Y direction, andthe scan speed in response to an instruction from the CPU 301. Further,the CPU 301 can identify the scan position of the observation lightbased on an output from the OCT scanner control circuit 311. The CPU 301can cause the fundus image capturing apparatus to function as describedbelow by reading and executing a program for implementing the processingillustrated in FIG. 4, the program being stored in the program storagedevice 304.

The SLO imaging processing is described. The CPU 301 sets apredetermined value to the SLO light source D/A converter 314 and sets apredetermined scan center position in the Y direction, a predeterminedscan width in the Y direction, and a predetermined scan speed to the SLOscanner control circuit 308. Thus, the CPU 301 controls the SLO scannercontrol circuit 308 to scan the fundus with the laser beam. At the sametime, the APD 110 outputs a signal proportionate to thereflection/scattering strength of the retina. The signal output from theAPD 110 is input to the CPU 301 via the APD interface 307. The CPU 301superimposes the output signal from the APD 110 at the scan position ofthe laser beam, so that a fundus image can be acquired. The CPU 301causes the display device 302 to display the fundus image.

The OCT imaging processing is described. The CPU 301 sets to the OCTscanner control circuit 311 the scan center positions in the X directionand the Y direction, the scan widths in the X direction and the Ydirection, the scan speed, and a main scanning direction, and causes theOCT scanner control circuit 311 to scan the retina with the observationlight from the OCT unit 115. At this point, the output of theone-dimensional sensor 213 in the OCT unit 115 is input to the CPU 301via the one-dimensional sensor interface 306. The CPU 301 performsprocessing such as frequency-to-wave number conversion fast Fouriertransform (FFT) in the main storage device 303 to acquire a depthdirection of the retina. The CPU 301 can acquire a three-dimensionalretinal image using the depth direction of the retina and the scanposition of the observation light, and causes the display device 302 todisplay the three-dimensional retinal image.

The processing performed by the control unit illustrated in FIG. 3 isdescribed with reference to the flowchart in FIG. 4. In step S401, theCPU 301 captures a fundus image by the SLO imaging processing. Morespecifically, the CPU 301 sets the predetermined value to the SLO lightsource D/A converter 314, sets the predetermined scan center position inthe Y direction, the predetermined scan width in the Y direction, andthe predetermined scan speed to the SLO scanner control circuit 308, andcauses the SLO scanner control circuit 308 to capture a fundus image.FIG. 5 illustrates an example of a fundus image acquired by the SLOimaging processing. In the fundus image illustrated in FIG. 5, the scanwidths in the X direction and the Y direction are about 7 mmrespectively. An examiner sets an imaging position of the fundus usingsuch a fundus image.

Then, the CPU 301 extracts a feature point on the eye. The number of thefeature points can be one when tracking is performed only in the Xdirection and the Y direction, however, the use of a plurality offeature points can increase the accuracy, and allows correction such asrotation. In the present exemplary embodiment, the tracking is performedin the X direction and the Y direction using two feature points. In theexample in FIG. 5, branch points 501 and 502 of blood vessels are usedas the feature points. The method for extracting the feature points isdiscussed in the above-described Japanese Patent Application Laid-OpenNo. 2001-70247, and accordingly, the description of the method isomitted.

For the tracking, an area 503 containing these feature points is to bescanned. However, due to movement referred to as the involuntary eyemovement during fixation, a larger area is to be scanned. FIG. 6illustrates typical movement of a human eye. As illustrated in FIG. 6,in a case where the imaging takes 15 seconds or more, for example, thescanning area is to be expanded by ±1.4 mm in the X direction and the Ydirection. The expanded area is an area 504. The CPU 301 determines thearea 504 as the scanning area. As compared to the scan area at the timeof the capturing the fundus image, the scanning area in the tracking issmall. Thus, when the scan is performed at the same frame rate, the scanspeed in the tracking can be reduced. Consequently, to irradiate thefundus at the same luminance in the capturing of the fundus image and inthe tracking, the light amount in the tracking can be reduced accordingto an area ratio between the scanning area in the capturing of thefundus image and the scanning area in the tracking.

In step S402, the CPU 301 starts the OCT imaging processing. From thispoint of time, the SLO imaging processing and the OCT imaging processingare simultaneously performed. First, the SLO imaging processing fortracking is described.

In step S403, the CPU 301 changes the scan center point in the Ydirection and the scan width in the Y direction with respect to the SLOscanner control circuit 308 to values corresponding to the scanning area504 calculated in step S401. Further, the CPU 301 sets the light amountto a small value proportionate to the area of the scanning area 504.

In step S404, the CPU 301 performs one frame scan on the scanning area504. In step S405, the CPU 301 detects deviations from the previousfeature points using a pattern matching method or the like, and notifiesthe OCT imaging processing of the deviation values as correction valuesby communication (in step S411).

In step S406, the CPU 301 determines whether the SLO imaging processingends. If the SLO imaging processing has not ended (NO in step S406), theprocessing returns to step S404. Whereas if the SLO imaging processingends (YES in step S406), the processing ends.

The processing for changing the imaging area in step S403 is started inresponse to the start of the OCT imaging processing, however, thepresent exemplary embodiment is not limited to this configuration.Alternatively, the SLO imaging range can be changed after focusadjustment is ended or adjustment of the coherence gate position isended in the OCT imaging processing.

The SLO imaging range can be changed immediately after the focus in theSLO is appropriately set. In such a case, the CPU 301 controls the SLOscanner control circuit 308 such that the imaging range is to beexpanded once in several frames, and causes the display device 302 todisplay the acquired image of the expanded imaging range. By thisprocessing, the examiner can appropriately refer to a planer imageperpendicular to the optical axis of the fundus.

In another example, the observation by the SLO and the accurate imagingby the OCT can be simultaneously performed by determining the OCTimaging range after the start of the SLO imaging or changing the SLOimaging range in response to determination of the OCT imaging positionby specifying the center of the imaging position or the like. Further,in yet another example, if the tracking is accurately performed bychanging the SLO imaging range after a shooting button is pressed, andthe OCT imaging is performed, then the examiner can check the SLO imageat a higher frame rate.

The OCT imaging processing is described. In step S407, the CPU 301 setsto the OCT control circuit 311 the scan center positions in the Xdirection and the Y direction, the scan widths in the X direction andthe Y direction, and the main scanning direction. In step S408, the CPU301 acquires data of one line in the main scanning direction.

In step S409, the CPU 301 feeds back the correction value transmitted bythe communication (step S411) to the scan center position in the OCTimaging processing.

In step S410, the CPU 301 determines whether the OCT imaging processingends. If the OCT imaging processing has not ended (NO in step S410), theprocessing returns to step S408. If the OCT imaging processing ends (YESin step S410), the processing ends. When the OCT imaging processingends, in the determination in the next step S406, it is determined thatthe SLO imaging processing also ends.

As described above, according to the present exemplary embodiment, themovement of the fundus detected by the SLO imaging processing is fedback to the OCT imaging processing. Thus, the OCT imaging processingwith the corrected fundus movement can be performed.

In the above-described exemplary embodiment, it is assumed that theframe rate is constant, and the light amount in the SLO imagingprocessing in the tracking is reduced in proportion to the area of thescanning area. Alternatively, if the light amount in the OCT imagingprocessing is enough, the frame rate can be increased without reducingthe light amount. Accordingly, further accurate tracking can beperformed. Alternatively, both of the values can be controlled halfway,that is, the frame rate is slightly increased and the light amount iscorrespondingly reduced.

For example, if the deviations of the feature points detected in stepS405 are large (that is, the eye movement is large), the frame rate isincreased and the light amount is reduced correspondingly. On the otherhand, if the deviations of the feature points detected in step S405 aresmall (that is, the eye movement is small), the frame rate is reducedand the light amount is increased correspondingly.

It is possible to associate the magnitude of movement of the eye, thereflectivity of the eye, the light amount in the SLO imaging processing,the frame rate in the tracking, and an evaluation value of a finallyacquired image quality in a table. With reference to the table, forexample, when the movement of the eye is large, the frame rate isincreased to prioritize the accuracy of the tracking, and the imagequality can be increased. Meanwhile, when the reflectivity of the eye islow, the light amount is increased to prioritize the light amount in theSLO imaging, and the image quality can be increased.

In the above-described exemplary embodiment, the branch points of theblood vessels are used for the feature points. However, the subject canslightly see the branch points if infrared light is used. As a result,the visual fixation of the subject's eye may not be stable. To solvethis issue, the optic papilla can be used. The subject cannot see theoptic papilla, so that stable visual fixation can be expected.

In the above-described exemplary embodiment, the positional deviation isdetected by the pattern matching using the feature points. However, themethod is not limited to the above, the CPU 301 can employ a patternmatching using a degree of similarity of pixel values of an image. Theprocessing speed of the positional deviation detection processing usingthe feature points is fast. Consequently, it is preferable to use theprocessing when the processing capability is poor, or when the framerate of the SLO moving image capturing is relatively high. On the otherhand, the positional deviation detection processing using the degree ofsimilarity is accurate. Consequently, it is preferable to use theprocessing when the processing capability is high and the frame rate isrelatively low.

In the above-described exemplary embodiment, sometimes it takes severalseconds to capture images, for example, when a high-resolutionthree-dimensional fundus image is acquired. During the processing, thedisplay of the fundus image is not updated. To solve this issue, the SLOimaging processing for acquiring a fundus image in step S401 can beperformed once in a plurality of times of the processing of the SLOimaging for tracking.

More specifically, the CPU 301 controls the SLO scanner control circuit308 such that the SLO imaging range is expanded to the imaging rangebefore the start of the tracking once in several frames and imagecapturing processing is performed. Further, the CPU 301 updates the SLOimage displayed on the display device 302 with the SLO image captured byexpanding the imaging range. Due to the change of the imaging range, theconstant frame rate in the SLO is lost, however, the fundus image isdisplayed on the display device 302 at a repetition rate smaller than atleast an average repetition rate in the SLO imaging. By this processing,the fundus image for observation is appropriately captured anddisplayed. Thus, the state of the fundus can be displayed in real timewhile the tracking accuracy is increased.

In the above-described exemplary embodiment, both of the control formaking the imaging area in the tracking smaller than the imaging area ofthe fundus image for observation, and the control for reducing the lightamount in the tracking than the light amount in the capturing of thefundus image for observation are performed. However, either one of thecontrol can be performed. For example, if the CPU 301 performs thecontrol such that the imaging area is set to a small area and the lightamount is not changed, the S/N ratio of the SLO image for tracking canbe maintained at a high level. Alternatively, if the CPU 301 performsthe control such that only the light amount is reduced and the imagingrange is not changed, the imaging area used for the positional deviationdetection can be expanded.

Further, the CPU 301 can determine whether to respectively perform thesetting of reducing the imaging area and the setting of reducing thelight amount according to an input of the user via an operation unit(not illustrated), and the CPU 301 can determine to perform both of theprocesses respectively. By this processing, the tracking accuracy andthe S/N ratio of the image can be controlled as required by the user,and an image desired by the user can be acquired.

In another exemplary embodiment, in a case where an SLO image and an OCTimage captured prior to a main image capturing in the OCT are provided,the CPU 301 calculates image qualities of the images using an index suchas an S/N ratio, and determines whether the quality of the OCT image inthe main image capturing is enough in consideration of superimposition,noise removal processing, and the like. As a result of thedetermination, if an adequate image quality can be ensured, the settingof reducing the imaging area of the SLO image for tracking and thesetting of reducing the light amount are not performed. By thisprocessing, the tracking accuracy can be maintained, and a high-qualityimage can be acquired.

In the above-described exemplary embodiments, the light amount fortracking can be reduced. Consequently, the reduction of the light amountfor the OCT imaging processing can be suppressed to a minimum, and thedeterioration of the fundus image can be prevented. Further, at the sametime, the tracking rate can be increased, and consequently, the trackingaccuracy can be increased.

Embodiment of the present invention can also be realized by a computerof a system or apparatus (or devices such as a CPU or an MPU) that readsout and executes a program recorded on a memory device to perform thefunctions of the above-described embodiments, and by a method, the stepsof which are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiments. For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the embodiment of the present invention has been described withreference to exemplary embodiments, it is to be understood that theinvention is not limited to the disclosed exemplary embodiments. Thescope of the following claims is to be accorded the broadestinterpretation so as to encompass all modifications, equivalentstructures, and functions.

This application claims priority from Japanese Patent Application No.2011-155870 filed Jul. 14, 2011, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An imaging control apparatus capable of causing afirst imaging unit to capture a fundus image of a subject's eye andcausing a second imaging unit to capture a tomographic image of thesubject's eye, the imaging control apparatus comprising: a displaycontrol unit configured to display a first fundus image on a displayunit, the first fundus image being captured by the first imaging unit ina first area on a fundus of the subject's eye; an extraction unitconfigured to extract one or more feature points from the first fundusimage; an area determination unit configured to determine a second areaon the fundus of the subject's eye, the second area including the one ormore feature points and being smaller than the first area; a controlunit configured to cause the first imaging unit to capture a secondfundus image of the subject's eye in the second area; a detection unitconfigured to detect a positional deviation of the fundus of thesubject's eye by comparing the first fundus image and the second fundusimage; and a correction unit configured to correct an image capturingposition of the second imaging unit for capturing the tomographic imagebased on the detected positional deviation.
 2. The imaging controlapparatus according to claim 1, wherein the control unit causes thefirst imaging unit to capture the second fundus image of the subject'seye in the second area, and in a light amount lower than a light amountused for the image capturing of the first fundus image in the firstarea.
 3. The imaging control apparatus according to claim 1, furthercomprising a light control unit configured to cause the first imagingunit to capture a first fundus image of the subject's eye in a firstlight amount and a second fundus image of the subject's eye in a secondlight amount, the second light amount being lower than the first lightamount.
 4. The imaging control apparatus according to claim 1, furthercomprising a first control unit configured to control at least one of aframe rate and a light amount at image capturing by the first imagingunit based on the positional deviation detected by the detection unit.5. The imaging control apparatus according to claim 1, furthercomprising a second control unit configured to control a light amount atimage capturing by the first imaging unit based on reflectivity of thefundus of the subject's eye.
 6. The imaging control apparatus accordingto claim 1, wherein the detection unit detects a position of an opticpapilla of the subject's eye as the position of the feature point. 7.The imaging control apparatus according to claim 1, wherein a part of aplurality of image capturing processes for detecting the positiondeviation by the detection unit is performed as an image capturingprocess for capturing the fundus image of the subject's eye in the firstarea.
 8. The imaging control apparatus according to claim 1, wherein thefirst imaging unit is a scanning laser ophthalmoscope.
 9. The imagingcontrol apparatus according to claim 1, wherein the second imaging unitis an optical coherence tomography apparatus.
 10. An ophthalmic imagingapparatus comprising: an imaging control apparatus according to claim 1,the first imaging unit; and the second imaging unit.
 11. The imagingcontrol apparatus according to claim 1, further comprising a repeatcontrol unit configured to repeat capturing the second fundus image bycontrolling of the control unit, detecting the positional deviation bythe detection unit and correcting the image capturing position by thecorrection unit.
 12. An imaging control apparatus capable of causing afirst imaging unit to capture a first fundus image in a first area of asubject's eye and causing a second imaging unit to capture a tomographicimage of the subject's eye, the imaging control apparatus comprising: anextraction unit configured to extract one or more feature points fromthe first fundus image; an area determination unit configured todetermine a second area on the fundus of the subject's eye, the secondarea including the one or more feature points and being smaller than thefirst area; a control unit configured to cause the first imaging unit tocapture a second fundus image of the subject's eye the second area; adetection unit configured to detect a movement of the subject's eye bycomparing the first fundus image and the second fundus image; and adetermination unit configured to determine an image capturing positionof the second imaging unit for capturing the tomographic image based onthe detected movement.
 13. The imaging control apparatus according toclaim 12, further comprising a repeat control unit configured to repeatcapturing the second fundus image by controlling of the control unit,detecting the positional deviation by the detection unit and determiningthe image capturing position by the determination unit.
 14. A method forcontrolling image capturing performed by an imaging control apparatuscapable of causing a first imaging unit to capture a fundus image of asubject's eye and causing a second imaging unit to capture a tomographicimage of the subject's eye, the method comprising: displaying a firstfundus image on a display unit, the first fundus image being captured bythe first imaging unit in a first area on a fundus of the subject's eye;extracting one or more feature points from the first fundus image;determining a second area on the fundus of the subject's eye, the secondarea including the one or more feature points and being smaller than thefirst area; causing the first imaging unit to capture a second fundusimage of the subject's eye in the second area; detecting a positionaldeviation of the fundus of the subject's eye by comparing the firstfundus image and the second fundus image; and correcting an imagecapturing position of the tomographic image of the second imaging unitfor capturing the tomographic image based on the detected positionaldeviation.
 15. The method according to claim 14, further comprising:causing the first imaging unit to capture a first fundus image of thesubject's eye in a first light amount and a second fundus image of thesubject's eye in a second light amount, the first light amount beinglower than the second light amount.
 16. A program for causing a computerto execute a method for controlling image capturing performed by animaging control apparatus capable of causing a first imaging unit tocapture a fundus image of a subject's eye and causing a second imagingunit to capture a tomographic image of the subject's eye, the programcomprising: displaying a first fundus image on a display unit, the firstfundus image being captured by the first imaging unit in a first area ona fundus of the subject's eye; extracting one or more feature pointsfrom the first fundus image; determining a second area on the fundus ofthe subject's eye, the second area including the one or more featurepoints and being smaller than the first area; causing the first imagingunit to capture a second fundus image of the subject's eye in the secondarea; detecting a positional deviation of the fundus of the subject'seye by comparing the first fundus image and the second fundus image; andcorrecting an image capturing position of the tomographic image of thesecond imaging unit for capturing the tomographic image based on thedetected positional deviation.
 17. The program according to claim 16,further comprising: causing the first imaging unit to capture a firstfundus image of the subject's eye in a first light amount and a secondfundus image of the subject's eye in a second light amount, the firstlight amount being lower than the second light amount.